As we approach the start of Atlantic hurricane season, a term you may see pop up now and then is “Accumulated Cyclone Energy.” Accumulated Cyclone Energy (ACE) is a measurement of the activity of a hurricane season, based on storm intensity and duration. It is calculated for each individual storm by taking the sum of the storm’s intensity, in knots, squared, at six-hour intervals, divided by 10000. For instance, if a storm had 40-knot winds for one advisory, the ACE for that advisory would be 0.16. The sum of the Accumulated Cyclone Energy values for all storms in a season is the total seasonal Accumulated Cyclone Energy. The concept of Accumulated Cyclone Energy may sound complicated to some, but it offers meteorologists a very valuable tool to accurately assess how active a hurricane season really is.

The most Accumulated Cyclone Energy ever recorded in an Atlantic hurricane season since 1950 occurred in the hyperactive 2005 season, which had 250 ACE – more than 2.5 times as many ACE points as a “normal” season. On average, an Atlantic hurricane season has just under 100 ACE. In the East Pacific basin, a record high 295 ACE was observed during the 1992 season, with an average of about 120 ACE. Long-lived or intense storms typically produce the most ACE. For instance, Category 5 Hurricane Matthew in the 2016 Atlantic hurricane season produced 50.9 ACE – the most of any storm since Hurricane Ivan in 2004.  These storms are often referred to as “ACE producers” and help increase the overall activity of a season.

Accumulated Cyclone Energy is not calculated for tropical depressions. Regardless of the wind speed of a tropical depression, ACE only applies if winds are 35 knots or greater. Prior to 2012, it was excluded for subtropical cyclones as well. Since 2012, subtropical cyclones do count to the overall ACE of a season. Even though several named storms may occur outside of season peak, the “bulk” of the ACE is typically produced during the peak season period (July through October for the East Pacific, and August 15 through October for the Atlantic). Storms outside of the season peak period typically do not produce much in the way of ACE – for instance, Tropical Storm Arlene in April only added 0.565 ACE to the season total.

The truth is the number of storms in a season alone can be misleading to measure the season’s overall activity. For instance, the 2013 Atlantic hurricane season had 14 named storms (above the long-term average of 12), but a mere 36 ACE – the lowest value since 1994. In contrast, the 1999 Atlantic hurricane season had 12 named storms (right near the long-term average), but 177 ACE – “hyperactive” on the ACE scale. For overall activity, Accumulated Cyclone Energy is a much more accurate measurement.

As the Atlantic and East Pacific hurricane seasons progress, I will periodically monitor the ACE and I thought it would be helpful to give a background on what it is and how it is calculated before the season starts. I will be back with another post tomorrow.